Closed Brayton-cycle (hereinafter "CBC") engines provide well known advantages over open-cycle engines, including high efficiency over a relatively wide power range. The latter advantage is normally realized by changing the inventory of the working fluid. The change in inventory is effected by increasing or decreasing the total mass of a working fluid of fixed composition. This known method of increasing engine output from part-power to full power is illustrated in FIG. 1. In the illustrated scheme, heat transfer rates and temperatures are maintained constant at various stages of the engine cycle to obtain maximum efficiency. Pressure variations are effected by injecting additional working fluid into the flow path or extracting working fluid therefrom. This produces a change in mass flow throughout the cycle, and provides the capability of varying engine output power.
The ability to achieve high efficiency over a relatively wide power range makes CBC engines attractive in applications which demand considerably different levels of output power but provide only a limited fuel supply. An exemplary application is a torpedo propulsion system that operates under two conditions of output power to accommodate the cruise and dash phases of the torpedo mission. Both phases of the mission typically consume a large fraction of the available fuel, and any energy savings which can be achieved for the cruise phase is available for consumption in the dash phase or for temporal extension of the cruise phase. Accordingly, the power turndown ratio of the engine (i.e. the maximum power for the given design divided by the minimum power at which high efficiency is maintained) has important tactical consequences in such an application.
An objective of the present invention is to increase power turndown ratios in the forementioned application for CBC engines by a factor of at least 2.0.